Sofia is not too young to wear heels. I was wearing heels like that when I was her age. It's not like she's in a pair of Louboutins with a 5-inch heel.
I thought she looked charming in all her looks, especially the sparkly dark blue dress. She was 100% age appropriate. Maybe we're as a society too used to seeing young ladies her age dressed far too adult, that when we see a 15-year-old dressed like a 15-year-old, we immediately say she's dowdy or dressed too old for her age. The problem isn't with her wardrobe, it's with us.
Sofia is also tall and taller than her sister, and considering she's still growing, she's probably in that awkward stage where the hemlines don't look right all the time, but once she's at her final height, she'll figure it out. We all have to. I'm 5'2, so I'd kill (not literally LOL) for her height.